1. Field of the Invention
The subject invention relates to methods and apparatus for producing interference fringe patterns for holograms, to hologram interferometry, and to real-time visualization of vibration patterns.
2. Information Disclosure Statement
The disclosure statement is made pursuant to the duty of disclosure imposed by law and formulated in 37 CFR 1.56(a). No representation is hereby made that information thus disclosed in fact constitutes prior art, inasmuch as 37 CFR 1.56(a) relies on a materiality concept which depends on uncertain and inevitably subjective elements of substantial likelihood and reasonableness, and inasmuch as a growing attitude appears to require citation of material which might lead to a discovery of pertinent material though not necessarily being of itself pertinent. Also the following comments contain conclusions and observations which have only been drawn or become apparent after conception of the subject invention or which contrast the subject invention or its merits against the background of developments subsequent in time or priority.
In holography, the spatial frequency or fringe spacing is proportional or inversely proportional, respectively, to the angle between the object and reference beams at the hologram. Especially if holograms of large objects are taken, the angle between object beam and reference beam varies across the hologram, whereby the hologram is in effect sensitive to object size.
Reference may in this respect be had to articles entitled The Non-Stroboscopic Visualisation of Vibration Patterns by Real Time--Time Averaged Hologram Interferometry, by P. Waddell and W. McCracken, Proc. ElectroOptical Systems Design Conf., Brighton (1972), pp. 14 to 22, The Study of Vibration Patterns Using Real-Time Hologram Interferometry, by W. F. Fagan, P. Waddell and W. McCracken, Optics and Laser Technology (August 1972), pp. 167 to 172, and A Hologram Interferometer with a Retro-Reflected Speckle Reference Beam for the Real Time Visualization of Vibration Patterns, by W. F. Fagen and P. Waddell, Proc. Brit. Acoustical Society, Vol. 2, No. 3 (Winter 1973).
In these and other holographic systems, the light-responsive medium on which holograms are made has a certain bandwidth of optimum spatial frequency response.
This is distinctly the case with thermoplastic hologram recording media where the finite thickness of the thermoplastic film limits the flow of thermoplastic material available for recording purposes. Reference may in this respect be had to the article entitled Thermoplastic Xerographic Holography, by J. C. Urbach and R. W. Meier, APPLIED OPTICS, Vol. 5, No. 4 (April 1966), pp. 666/67, and to U.S. Pat. Nos. 3,560,205 and 3,655,257, by J. C. Urbach, disclosing methods and apparatus for forming a phase hologram on a deformable thermoplastic. Similar frequency limitations or sensitivities are observed with holographic recording media of a silver-halide or other photographic type, and also in holographic systems employing video cameras or other video systems for observation and similar purposes.
Accordingly, holographic systems of the subject type suffer degradation, if recorded or observed spatial frequencies go beyond the optimum bandwidth of the recording or observation medium.
The presence of wavelengths or spatial frequencies outside the optimum range of the holographic recording or light-responsive medium, as well as a superposition of a broad range of spatial frequencies at any spot on the medium, as would be caused by an angular extent of the object beam with respect to that spot, leads to objectionable signal-to-noise ratios. This is particularly the case when a large object size leads to angular incidence variations across the recording or light-responsive medium. In laboratory terminology, the hologram then has to "work harder" in order to provide the desired image or pattern. This means, in so many words, that a relatively high amount of energy is required to reproduce the holographic image, since a high proportion of the energy is being expended to reconstruct a multitude of rays, and this extensive activity generates a proportionate level of optical noise.